Are Pterosaurs Dinosaurs? The Flying Reptiles That Weren’t

In almost every dinosaur film and picture book, flying reptiles soar over the heads of Tyrannosaurus and Triceratops. We often call them dinosaurs too. But scientists don’t. Pterosaurs were a separate group of animals, and their story is just as remarkable.

Cousins, not dinosaurs

To be a dinosaur, an animal has to belong to a particular branch of the reptile family tree, defined by its ancestry and by features such as the shape of the hip and the way the legs are held under the body. Pterosaurs sit on a neighbouring branch.

Both belong to the archosaurs, the “ruling reptiles”, which also include crocodiles. Within that group, dinosaurs and pterosaurs are thought to be close relatives, but each forms its own lineage. The same goes for the great sea reptiles of the time: plesiosaurs, ichthyosaurs and mosasaurs were not dinosaurs either.

The first vertebrates to fly

Pterosaurs first appear in the fossil record about 225 million years ago, in the Late Triassic, around the same time as the first dinosaurs. They were the first animals with backbones to evolve powered, flapping flight. Only insects did it earlier. Birds came tens of millions of years later, and bats much later still.

A wing made from one finger

The pterosaur wing was unique. Each wing was a membrane of skin and muscle stretched between the body, the hind leg and a single, enormously long fourth finger. The other three fingers stayed small, with claws for gripping. Inside the membrane were stiffening fibres that helped control its shape.

That is very different from the other flyers:

• Bird wings are made of feathers attached to the arm and a fused hand.
• Bat wings are skin stretched between several long fingers.

Pterosaurs also had extremely light, hollow bones with thin walls, and air sacs similar to those of birds and many dinosaurs.

From sparrows to giraffes

Pterosaurs came in an astonishing range of sizes. Some had wingspans of less than half a metre. At the other extreme, Quetzalcoatlus, which lived in North America at the very end of the Cretaceous, had a wingspan of about 10–11 metres, like a small aircraft. Standing on the ground on all fours, it was as tall as a giraffe.

How could something so big get airborne? In 2010, palaeontologists Mark Witton and Michael Habib argued that giant pterosaurs could still fly well. Rather than running and flapping like a bird, they probably used a quadrupedal launch: crouching on all fours and vaulting into the air with their powerful arms, which carried the wings, before the first downstroke.

Fuzzy, colourful and doomed

Pterosaur fossils show that many were covered in fine filaments called pycnofibres, giving them a fuzzy coat. Many species had striking head crests, probably used for display. They ate fish, insects and small animals, and some may have hunted on the ground like storks.

Like the non-bird dinosaurs, pterosaurs disappeared in the mass extinction 66 million years ago. Today, the only flying descendants of that ancient reptile world are the birds, which are dinosaurs themselves.

In short

• Pterosaurs were flying reptiles, but they were not dinosaurs.
• Both groups are archosaurs, like crocodiles, but on separate branches.
• Pterosaurs were the first vertebrates to evolve powered flight, about 225 million years ago.
• Their wings were skin stretched from one hugely long fourth finger.
• Quetzalcoatlus had a wingspan of about 10–11 metres and was as tall as a giraffe.
• Pterosaurs died out 66 million years ago, along with the non-bird dinosaurs.